Method for producing platinum group metal or platinum group-based alloy

ABSTRACT

A method for producing a platinum group metal or a platinum group-based alloy according to the present invention includes a preparing step of weighing a raw material that is partially or entirely of powder, a molding step of molding and solidifying the prepared raw material to obtain molded bodies, a sintering step of sintering the molded bodies to obtain a sintered body, a melting step of melting the sintered body to produce a molten ingot, and a deformation processing step of processing the molten ingot. In the sintering step, the molded bodies are sintered in a stacked state to produce a sintered body as a joined body.

TECHNICAL FIELD

The present invention relates to a method for producing a platinum groupmetal or a platinum group-based alloy, and more particularly, toproduction of a molten in got in a method for producing a platinum groupmetal or a platinum group-based alloy.

BACKGROUND ART

A platinum group metal or a platinum group-based alloy is designed usingheat resistance, oxidation resistance, and chemical resistance of theplatinum group metal, and is widely used as a high temperature member ora corrosion-resistant product. The platinum group metal as used hereincollectively refers to Pt, Pd, Rh, Ir, Ru, and Os.

Processes for production of a platinum group metal or a platinumgroup-based alloy generally include a compounding step, a melting step,a deformation processing step, and the like for an alloy raw material.In the melting step, a melting method for producing a molten ingot canbe classified into several types. A platinum group metal as a maincomponent has a very high melting point (1,500° C. or higher), and hencean induction heat melting furnace or an energy beam melting furnacehaving a melting ability of 2,000° C. or higher is used.

Energy beam melting includes nonconsumable arc melting, vacuum plasmamelting, atmospheric pressure plasma arc melting, electron beam melting,and the like, and those melting processes are common in being performedthrough radiation of an energy beam to a raw material in a water-cooledcopper crucible. The molten raw material is in the form of a plate, awire, powder, or the like including an ingot and scrap, and isappropriately compounded by a predetermined amount for use.

The energy beam melting is broadly divided into two types according tomethods employed in the water-cooled copper crucible. One method is theuse of a boat-shaped water-cooled copper crucible. The boat-shapedwater-cooled copper crucible is a water-cooled copper crucible having acavity (hollow) in the shape of a circle, a rectangle, or the like, anda whole quantity of a raw material placed in the cavity is molten toobtain a molten ingot (Patent Literature 1).

Another method is the use of a water-cooled copper crucible having athrough cavity. In this method, while a raw material rod as the rawmaterial is held horizontally, one end of the raw material rod isexposed to an energy beam to be molten, and the molten metal iscontinuously dropped, to thereby form a molten pool in the cavityreceiving the dropped molten metal. A bottom portion of the molten poolis continuously pulled down to obtain a rod-like molten ingot (PatentLiterature 2). The raw material rod is generally produced by melting theraw material in advance.

When the molten raw material is partially or entirely of powder, and theraw material in the form of powder is molten as it is, the raw materialis thrown up or scattered due to a flow of the energy beam. In somecases, in order to prevent the powder from being thrown up, the powderis subjected to compression molding in advance with a molding methodsuch as press molding or CIP mold (Patent Literature 3).

In the powder subjected to the compression molding, particles arebrought into contact with each other and entangled to form a unitedappearance. Thus, even when an energy beam is radiated to the particles,the particles are prevented from being blown off and thrown up. A moldedbody generally has a relative density of from about 30% to about 60% andincludes voids to a considerable extent, and an atmospheric gas or aresidual gas exists in the voids.

Further, the molded body merely has a united appearance. Thus, themolded body is easily broken down due to an impact caused by beingdropped or other factors, and also causes powder that exfoliates from asurface of the molded body during conveyance, thereby degrading amaterial yield. The material yield as used herein refers to a ratio of amass of the molten ingot to a mass of the molten raw material.

Incidentally, when an energy beam is radiated to the molded body, themolded body is heated with conducted heat, radiated heat, and Jouleheat, and a temperature of the molded body abruptly rises mainly at anirradiated portion thereof. At this time, the gas existing in the voidsabruptly expands, and hence the particles merely united in appearanceare flicked to the outside of the water-cooled copper crucible. At thesame time, the partly molten metal is also flicked out. As a result, themass of the molten ingot is reduced accordingly. That is, the materialyield is degraded, thereby causing a large economic loss in productionof a very expensive platinum group metal.

Further, when the raw material is a powder mixture, the material yieldis degraded, and the composition may also be changed. When the moldedbody is broken to cause a piece thereof to be dropped, cause powder onthe surface of the molded body to exfoliate, or cause the molten metalto scatter while being molten, a component contained in that portion isnot included in the molten ingot, and hence accurate alloy compositioncannot be obtained. Further, in the energy beam melting using theboat-shaped water-cooled copper crucible, an energy beam for melting isradiated from above. However, the raw material is generally turnedupside down and molten also from an opposite side and this operation isrepeated to obtain a molten ingot having uniform composition. At thistime, a dropped piece and exfoliated powder may be left unmolten at acorner of the boat-shaped cavity in the water-cooled copper hearth. Sucha case may also prevent the alloy composition from being accurate.

CITATION LIST Patent Literature

-   [PTL 1] JP 2002-105631 A-   [PTL 2] JP 2009-93881-   [PTL 3] JP 2004-137580

SUMMARY OF INVENTION Technical Problem

The present invention has been made in view of problems of the relatedart described above, and an object of the present invention is toprovide a method for producing a platinum group metal or a platinumgroup-based alloy having a high material yield.

Solution to Problem

According to the present invention, provided is a method for producing aplatinum group metal or a platinum group-based alloy, including: apreparing step of weighing a raw material that is partially or entirelyof powder and, when the alloy is to be produced, mixing the weighed rawmaterial to obtain a powder mixture; a molding step of molding andsolidifying the prepared raw material to obtain molded bodies; asintering step of sintering the molded bodies in a furnace to obtain asintered body as a joined body; a melting step of melting the sinteredbody to produce a molten ingot; and a deformation processing step(plastic working step) of processing the molten ingot, in which, in thesintering step, the molded bodies are sintered in the furnace in astacked state to produce the sintered body having a relative density of70% or more.

The preparing step is a step of weighing the raw material in accordancewith a desired amount of the molten ingot. When the alloy is to beproduced, each raw material is weighed so as to obtain a predeterminedalloy composition. The raw material may have any shape. However, the rawmaterial is at least partially or entirely of powder.

The molding step is a step of molding and solidifying the raw material,which is partially or entirely of powder in the whole quantity of theraw material, to obtain the molded bodies, and a well-known dry moldingmethod such as uniaxial pressing, tableting, cold isostatic pressing(CIP), or rubber pressing is suitable therefor. The shape can beappropriately selected from among the shape of a disc/cylinder, theshape of a sharp-edged plane figure including a polygon/prism, abriquette having no regular shape, and the like. The number of themolded body can be determined depending on a shape and dimensions of thewater-cooled copper crucible, and may be one or more.

The sintering step is a step of sintering to substantially unify themolded bodies. A well-known sintering furnace (firing furnace) such as agas furnace or an electric furnace can be used therefor, and both abatch type and a continuous type are suitable. A sintering temperaturecan be appropriately selected depending on the kind of the raw material,but a range that is 1,200° C. or higher and does not exceed a meltingpoint of the raw material is more suitable for a platinum group metal ora platinum group-based alloy having a melting point that is above 1,500°C. An atmosphere, an inert gas, or a vacuum can be applied as asintering atmosphere, and are appropriately selected depending on thekind of the raw material. In the sintered body, individual particles areunified, thereby increasing the strength, and increasing the densitythrough sintering shrinkage. It is preferred that the sintered body hasa relative density of 70% or more.

With such a sintered body, particles are unified to increase thestrength, thereby preventing a piece of the molded body from droppingand the powder from being exfoliated. Further, occurrence of scatteringduring melting in the melting step can be suppressed, and hence a changein the alloy composition can also be suppressed.

As described above, when the molded body is used as a raw material rodin a melting step in a pull-down method, the molded body may be brokenduring melting due to insufficient strength. Further, the molded bodymay collapse with a small force. Thus, there is a difficulty in graspingthe molded body with a raw material rod feeding mechanism and using themolded body as it is. According to the present invention, particles areunified through sintering, and a high strength is obtained. Thus, themolded body can be used as a raw material rod without the fear ofbreakage and collapse.

In the sintering step, a plurality of molded bodies can be sintered in astacked state into the sintered body as a joined body. The shape of themolded body can be appropriately selected from among the shape of adisc/cylinder, the shape of a sharp-edged plane figure including apolygon/prism, and the like. Specifically, in the sintering step, whenthe molded bodies are sintered in a stacked state, particles in theindividual molded bodies and also particles in contact with each otherat an interface between the stacked molded bodies are sintered andunified. In such a manner, a rod-like sintered body (joined body) can beobtained. There is an advantage in that, through appropriate selectionof dimensions and the number of the molded bodies to be stacked, changescan be made as necessary from a very small and short raw material rod toa long raw material rod. In particular, the rod-like sintered body issuitable for use as a raw material rod in a melting step of thepull-down system.

With regard to a related-art raw material rod, before a melting step, amolten ingot is produced in advance in an energy beam melting furnaceusing a boat-shaped water-cooled copper crucible, and the molten ingotis used as a long raw material rod. The molten ingot produced in such amanner has no regular shape. Specifically, the shape of the boat-shapedwater-cooled copper crucible is transferred to a bottom portion of themolten ingot and thus, the bottom portion has a regular shape, but aside surface and an upper surface of the molten ingot are in the shapeof the solidified molten metal as it is. When a latent heat at constantvolume in the melting is high as in the case of a platinum group metalor a platinum group-based alloy, the molten metal is liable to besolidified immediately after the molten metal separates from the energybeam (heat source). Thus, occurrence of a burr on a side surface andwaviness on the upper surface of the molten ingot are conspicuous, andthe raw material rod has an indefinite sectional area. The latent heatat constant volume (kJ/cm³) as used herein is latent heat necessary formelting a substance per unit volume, and is defined by heat of fusion(kJ/mol), molar mass (g/mol), and density (g/cm³).

When pull-down melting is performed with such a raw material rod, thereis a difficulty in dripping the molten metal at a constant speed. Thus,at a portion having a small sectional area, the molten metal to bedripped becomes insufficient, and hence a defect such as a pore isliable to occur in the molten ingot. At a portion having a largesectional area, the molten metal to be dripped becomes excessive, andhence a trouble that the dripped molten metal overflows the cavity inthe water-cooled copper crucible and is solidified is liable to occur.

According to the present invention, molded bodies having fixeddimensions can be sintered in the sintering step, and the molded bodiescan be used as a raw material rod having fixed dimensions, and hencesuch problems do not arise. Further, manufacture of a raw material rodrequires a dedicated melting facility (melting furnace, crusible, andthe like). However, according to the present invention, such a facilityis unnecessary, and a general electric furnace or the like can be usedto produce the raw material rod (sintered body) very conveniently.

The molded body used for the raw material rod may have an appropriateshape. However, when the molded body is shaped to be substantiallyrectangular parallelepiped by uniaxial pressing, the molding isparticularly easy. Further, such a shape is very convenient in stackingthe molded bodies in the sintering step (claim 2).

Incidentally, a pressure in the furnace in the energy beam meltingdiffers depending on the melting method and the molten raw material(high vacuum to atmospheric pressure). In particular, an electron beammelting furnace requires a high vacuum region of 0.1 Pa or less. Whenthe vacuum is high as described above, a pressure difference with a gascomponent remaining in voids in the sintered body is large, and hencesubtle scattering may occur. Therefore, the suitable pressure in thefurnace in the melting is 1 Pa or higher.

The melting step is a step of producing the molten ingot using thesintered body as a raw material. Not only the energy beam meltingdescribed above but also related-art melting furnaces or melting methodswidely used for producing a platinum group precious metal and a platinumgroup-based alloy are applicable. For example, a sufficient inducedcurrent cannot be obtained through induction heat melting of a powderraw material due to a small contact area between particles. Thus, theinduction heat melting is regarded as inappropriate. However, accordingto the present invention, the particles are substantially unifiedthrough the sintering, and a sufficient induced current can be obtained.Thus, induction heat melting is also applicable.

Further, in the melting step, an energy beam melting furnace with thewater-cooled copper crucible having a through cavity is used. One end ofthe rod-like sintered body (joined body) as a raw material rod isexposed to an energy beam to be molten, and the molten metal iscontinuously dripped, to thereby form a molten pool in the cavityreceiving the dripped molten metal. A bottom portion of the molten poolis continuously pulled down to obtain the rod-like molten ingot.Specifically, it is suitable to use the rod-like sintered body (joinedbody) as a raw material rod in the melting step of the pull-down system.

Further, according to the present invention, provided is a method forproducing a platinum group metal or a platinum group-based alloy,including: a preparing step of weighing a raw material that is partiallyor entirely of powder and, when the alloy is to be produced, mixing theweighed raw material to obtain a powder mixture; a molding step ofmolding and solidifying the prepared raw materials to obtain moldedbodies, a sintering step of sintering the molded bodies to obtain asintered bodies, a melting step of melting the sintered body usingenergy beam melting that uses a boat-shaped water-cooled copper cruciblehaving a cavity formed therein to produce a molten ingot, and adeformation processing step (plastic working step) of processing themolten ingot. In the sintering step, a shape and dimensions of eachsintered body are determined so as to conform to the cavity, and, in themelting step, the sintered bodies conforming to the cavity are denselyarranged in the cavity of the boat-shaped water-cooled copper crucibleto produce the molten ingot.

In the boat-shaped water-cooled copper crucible in the energy beammelting, a circular or rectangular cavity (hollow) is generally formedin an upper surface of copper in which a water-cooling circuit isprovided. The molten raw material is placed in the cavity, and an energybeam is radiated from above to perform heating and melting. Throughdesigning the shape and the dimensions of the sintered body so as toconform to the cavity, more molten ingots can be obtained. Specifically,when a molded body in the shape of a cylinder or a disc is sintered andarranged in a circular cavity, and a molded body in the shape of a cube,a rectangular parallelepiped, or a hexagonal column is sintered andarranged in a rectangular cavity, two-dimensional dense arrangement ofthe molded bodies can be made, and hence stacking of the molded bodiesis also easy.

The deformation processing step is a step of processing the molten ingotinto a desired shape such as a plate or a wire, and a well-known methodis applicable thereto. Deformation processing of the molten ingotproduced according to the present invention can be performed as in thecase of a related-art molten ingot obtained through steps without thesintering step.

For example, when processing into a plate is performed, forging androlling are performed. When processing into a wire is performed,forging, groove rolling, and wire drawing are performed. In any of thecases, depending on the extent of work hardening, heat treatment isgiven as appropriate midway during the processing to perform softening.After the processing into a plate or a wire, depending on the intendeduse, processing such as cutting, bending, or welding can also beperformed. Further, with regard to each processing, both cold workingand hot working, in which a material is heated when processed, areapplicable.

Advantageous Effects of Invention

As described above, according to the producing method of the presentinvention, as compared to a related-art producing method, scattering ofa raw material during melting can be effectively suppressed, and thematerial yield of an expensive platinum group metal or an expensiveplatinum group-based alloy can be improved.

Further, as compared to the molded body, the sintered body has a higherstrength and is not broken easily, and hence powder can be preventedfrom being exfoliated during conveyance. Such feature is advantageous inthat part of the raw material is prevented from dropping or exfoliating,and hence change in composition does not occur. Further, there is alsoan advantage in that, when the sintered body is used as a raw materialrod, the sintered body can be supported and grasped in an apparatuswithout difficulty.

Further, there is also an advantage in that, as compared to the moldedbody, the sintered body has a higher density, that is, a smaller volumeper the same mass, and hence more raw material can be mounted on thewater-cooled copper crucible, thereby contributing to improvement inproductivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of an exemplary sintered body.

FIG. 2 is an illustration of another exemplary sintered body.

DESCRIPTION OF EMBODIMENTS

A method for producing an electrode chip of a spark plug for an internalcombustion engine is taken as an example and is described more indetail.

For an electrode chip of a spark plug, an iridium-base alloy or aplatinum-base alloy is preferred for use. In this example, a wholequantity of a raw material is of powder, and Ir powder and Pt powder areused.

(Raw Material Preparing Step)

Predetermined amounts of the respective powders are weighed so as toobtain predetermined composition, and a V-mixer is used to mix thepowders to obtain a uniform powder mixture.

(Molding Step)

The powder mixture is charged in a hopper of an automatic press formingmachine (uniaxial pressing). A rectangular cavity having short sides of20 mm and long sides of 50 mm is formed in a molding die, and fourcorners thereof have an R of 2 mm. The molded body is substantially inthe shape of a rectangular parallelepiped having dimensions of 20 mm×20mm×50 mm with corners thereof having an R of 2 mm (FIG. 1). A moldingpressure is 200 MPa. The molding pressure can be appropriately set, butis preferred to be approximately 120 MPa or higher. When the moldingpressure is 200 MPa or higher, a more highly dense molded body having arelative density of about 50% or more can be obtained. As the density ofthe molded body becomes higher, thermal energy necessary for thesintering can be reduced more, thereby being advantageous. However, anexcessively high density may cause breakage of the molded body. Otherthan this, through charging the powder mixture in a rubber hose,hermetically sealing the rubber hose, and performing CIP mold, a roundrod-like molded body can be obtained. Also in this case, a moldingpressure is preferred to be 120 MPa or higher, and a molding pressure ofabout 300 MPa is suitable.

When the two exemplary molded bodies are molten as they are as in therelated art, there can be visually recognized a state in which part ofthe heated powder and molten metal scatter in the melting furnace tocause sparkling. Further, the strength is to such an extent that a touchwith a hand may cause the powder to attach to a finger, and that acorner of the molded body is broken when the molded body is dropped froma height of about 5 cm.

(Sintering Step)

Five molded bodies are vertically stacked with surfaces of 20 mm×20 mmthereof being upper and lower surfaces, and these are counted as oneunit (FIG. 2). Four units are arranged in a setter formed of carbon, andthe molded bodies, together with the setter, are inserted into anatmospheric furnace including a carbon heater. Sintering is performed toobtain a sintered body at 1,200° C. or higher for 3 hours under argonairflow. The sintered body undergoes sintering shrinkage, with theresult that a raw material rod having a relative density of 70% or moreand dimensions of about 16 mm×16 mm×220 mm is obtained (FIG. 2).

(Melting Step)

The raw material rod is horizontally grasped by a raw material rodfeeding mechanism of an atmospheric pressure plasma arc melting furnace(pull-down system) and is continuously molten and dripped in an argonatmosphere of from 0.9 atm (atmospheric pressure) to 1.2 atm, and abottom portion of a water-cooled copper crucible is pulled down. Withthis, a cylindrical ingot having a diameter of φ 35 mm is obtained. Thestate of scattering is not observed during melting, and an effect of thesintering step can be confirmed. Further, although the raw material rodis in a cantilever state at this time, the raw material rod is notbroken, and powder does not exfoliate during the melting step.

(Deformation Processing Step)

The molten ingot is formed into a square rod by hot forging, and then,into a wire having a substantially rectangular section by hot grooverolling. The molten ingot is further formed into a round wire having apredetermined outer diameter by hot drawing using a die.

(Cutting Step)

The round wire is cut into lengths suitable for a wire saw. A pluralityof wires are arranged so as to be in parallel with one another, fixedwith a resin, and cut by the wire saw, to thereby obtain electrode chipsfor a spark plug each having a predetermined length.

EXAMPLES

Further description is given using Examples.

In Table 1, there are shown results. Evaluation was made in accordancewith the following criteria.

Reduction in mass represents reduction in mass of the molten ingot fromthe raw material powders at the time of the compounding, and isexpressed in percentage. The reduction in mass which is more than 3% wasdenoted by x, and 3% or less was denoted by ∘.

With regard to powder exfoliation, when a sintered body or a molded bodybefore being molten was picked up with fingers, and attachment of powderto the fingers was observed, the powder exfoliation was denoted by x.When attachment was not observed at all, the powder exfoliation wasdenoted by ∘.

With regard to a molten state, visual observation was made during themelting. When a sparkling-like scattering phenomenon was continuallyobserved, the molten state was denoted by x. When the phenomenon wasobserved once in a while, the molten sate was denoted by A. When thephenomenon was hardly observed, the molten state was denoted by ∘.

With regard to comprehensive judgment, these results were taken intoconsideration. When the effect of the present invention was notrecognized, the comprehensive judgement was denoted by x. When theeffect was recognized, the comprehensive judgement was denoted by ∘.When the effect was more considerable, the comprehensive judgement wasdenoted by ∘∘.

According to the description above (Description of Embodiments), moldedbodies were prepared with an automatic press forming machine (uniaxialpressing). A molding pressure was 200 MPa. Then, the molded bodies weresintered at different temperatures for three hours. Table 1 shows arelative density of each of the molded body and the sintered bodiesprepared at different sintering temperatures. The sintered bodies havinga relative density of 70% or more were obtained at sintering temperatureof 1200° C. or higher.

TABLE 1 Relation between Sintering Temperature and Relative DensitySintering Temperature (° C.) Molded Body 900° C. 1,100° C. 1,200° C.1,300° C. 1,500° C. Relative 52 55 60 71 74 74 Density (%)

Example 1

In Example 1, five molded bodies each having dimensions of 20 mm×20mm×50 mm were prepared. The five molded bodies were vertically stackedwith surfaces of 20 mm×20 mm thereof being upper and lower surfaces. Themolded bodies in a stacked state were arranged in a setter formed ofcarbon, and the molded bodies, together with the setter, were insertedinto an atmospheric furnace including a carbon heater. Sintering wasperformed to obtain a sintered body at 1,500° C. for 3 hours under argonairflow. With regard to the density of the molded body calculated fromthe dimensions and the mass, the relative density was 52%. The densityof the sintered body was 74%. Using the sintered body as the rawmaterial rod, a molten ingot having a diameter of about φ35 mm×a lengthof L 150 mm was manufactured.

When visual observation was conducted during the melting (under apressure of 1.1×10⁵ Pa), the scattering phenomenon was not at allobserved. The reduction in mass of the molten ingot from the rawmaterial preparing step was 0.6% or less. Further, after the sinteringuntil the melting was completed, the raw material rod was not broken orexfoliated.

Almost no scattered material was left in the furnace after the melting,and attachment of the scattered material to the water-cooled coppercrucible was not recognized.

Examples 2 and 3

In Example 2, the same procedure as in Example 1 was repeated exceptthat the sintering temperature was changed to 1300° C. In Example 3, thesame procedure as in Example 1 was repeated except that the sinteringtemperature was changed to 1200° C. The density of the sintered bodiesobtained in Examples 2 and 3 were 74% and 71%, respectively. Using thesintered body as the raw material rod, a molten ingot having a diameterof about φ35 mm×a length of L 150 mm was manufactured. In Examples 2 and3 as in the case of Example 1, when visual observation was conductedduring the melting (under a pressure of 1.1×10⁵ Pa), the scatteringphenomenon was not at all observed. The reduction in mass of the molteningot from the raw material preparing step was 0.6% or less. Further,after the sintering until the melting was completed, the raw materialrod was not broken or exfoliated.

Almost no scattered material was left in the furnace after the melting,and attachment of the scattered material to the water-cooled coppercrucible was not recognized.

Reference Example

In Reference Example, molded bodies were prepared as in the case ofExample 1. Each molded body was substantially in the shape of arectangular parallelepiped having dimensions of 20 mm×20 mm×50 mm withcorners having an R of 2 mm. Such molded bodies were individuallysintered without being stacked to obtain sintered bodies of about 16mm×16 mm×44 mm. The density of each sintered body calculated from thedimensions and the mass was 74%. The sintered bodies were placed on aboat-shaped water-cooled copper crucible and were molten by vacuumplasma melting to manufacture a molten ingot of about 15 mm×30 mm×100mm. The pressure during the melting was adjusted to be 5×10⁻¹ Pa (Ar).

In visual observation during the melting, occasional scattering wasobserved. A small amount of the scattered material was left in thefurnace after the melting, with part thereof being attached to thewater-cooled copper crucible.

The reduction in mass of the molten ingot was 2.5%. Further, with regardto the shape of the molten ingot, the bottom portion was approximatelysmooth along the shape of the boat-shaped water-cooled copper crucible.However, there was a burr on a side surface, and the upper surface wassolidified in a wavy state.

Comparative Example 1

In Comparative Example 1, after the raw material powder was mixed usinga V-mixer, the CIP method was used to manufacture a cylindrical moldedbody having a diameter of φ 30 mm. The molding pressure was 300 MPa.With regard to the density of the molded body calculated from thedimensions and the mass, the relative density was 48%. The molded bodywas cut into lengths of about 30 mm, placed on a boat-shapedwater-cooled copper crucible, and molten by arc melting to manufacture amolten ingot of about t 15 mm×w 30 mm×L 100 mm. The pressure during themelting was adjusted to 8×10⁴ Pa (Ar).

The strength of the molded body was not low to such an extent that atouch by a hand causes the molded body to be broken. However, when themolded body was taken out of a CIP die, powder attached to a finger, andpowder attached to an inner wall of the CIP die could be confirmed.

In visual observation during the melting, continuous scattering from amolten portion until the molded body was entirely molten away wasconfirmed. The scattered material was left in the furnace after themelting, and attachment thereof to the water-cooled copper crucible wasconspicuous. Further, the scattered material and powder that exfoliatedfrom the molded body were left in a corner of the bottom portion of theboat-shaped water-cooled copper crucible. As described above, part ofthe compounded raw material powders was left unmolten and the reductionin mass of the molten ingot from the raw material preparing step was3.2%.

With regard to the shape of the molten ingot, there were a burr andwaviness as in the case of Example 2.

Comparative Example 2

In Comparative Example 2, a molded body manufactured as in the case ofComparative Example 1 was placed on a boat-shaped water-cooled coppercrucible, and a molten ingot of about t 15 mm×w 30 mm×L 100 mm wasmanufactured by vacuum plasma melting. The pressure during the meltingwas adjusted to 5×10⁻¹ Pa (Ar).

In visual observation during the melting, continuous scattering from amolten portion until the molded body was entirely molten away wasconfirmed. More scattered material was left in the furnace after themelting, and attachment thereof to the water-cooled copper crucible wasmore conspicuous. Further, the scattered material and powder thatexfoliated from the molded body were left in a corner of the bottomportion of the boat-shaped water-cooled copper crucible. As describedabove, part of the compounded raw material powders was left unmolten,and the reduction in mass of the molten ingot from the raw materialpreparing step was 4.5%.

From the results described above, it was confirmed that, in a method ofdirectly melting a molded body without performing the sintering step,the reduction in mass was larger, and the material yield was lower,whereas, in melting the sintered body having a relative density of 70%or more according to the present invention, the reduction in mass wassmaller.

TABLE 2 Experimental Results Reduction Powder Molten Comprehensive No.in Mass Exfoliation State Judgment Example 1 ∘ ∘ ∘ ∘∘ Example 2 ∘ ∘ ∘ ∘∘Example 3 ∘ ∘ ∘ ∘∘ Reference ∘ ∘ Δ ∘ Example Comparative x x x x Example1 Comparative x x x x Example 2

1. A method for producing a platinum group metal or a platinumgroup-based alloy, comprising: a preparing step of weighing a rawmaterial that is partially or entirely of powder and, when the alloy isto be produced, mixing the weighed raw material to obtain a powdermixture; a molding step of molding and solidifying the prepared rawmaterial to obtain molded bodies; a sintering step of sintering themolded bodies in a furnace to obtain a sintered body as a joined body; amelting step of melting the sintered body to produce a molten ingot; anda deformation processing step of processing the molten ingot, wherein,in the sintering step, the molded bodies are sintered in the furnace ina stacked state to produce the sintered body having a relative densityof 70% or more.
 2. A method for producing a platinum group metal or aplatinum group-based alloy according to claim 1, wherein, in the moldingstep, the molded bodies are substantially in the shape of a rectangularparallelepiped.
 3. A method for producing a platinum group metal or aplatinum group-based alloy according to claim 1, wherein, in the meltingstep, a pressure during the melting is 1 Pa or higher.
 4. A method forproducing a platinum group metal or a platinum group-based alloyaccording to claim 3, wherein, in the melting step, a plasma arc meltingfurnace with a water-cooled copper crucible having a through cavity isused, a molten pool of a molten metal of the sintered body is formed inthe cavity, and a bottom portion of the molten pool in the cavity ispulled down to obtain the molten ingot.
 5. A method for producing aplatinum group metal or a platinum group-based alloy according to claim2, wherein, in the melting step, a pressure during the melting is 1 Paor higher.